Nerve conduction studies (NCS) are performed in order to detect and evaluate focal and systemic neuropathies of the peripheral nerves and the spinal nerve roots.
Carpal Tunnel Syndrome (CTS) is a common focal neuropathy caused by the compression of the median nerve at the wrist. Symptoms of mild CTS include wrist pain, tingling and numbness, while severe CTS can result in the inability to use one's hand. Focal neuropathies typically cause degradation of nerve and muscle function at the focal location and distal to the focal location. Such degradation typically causes nerve conduction to be slowed or blocked in the focal nerve segment.
Diabetic polyneuropathy (DPN) is a systemic neuropathy primarily affecting the peripheral nerves of the lower extremities. Systemic neuropathies typically degrade nerve function over substantially the entire length of the peripheral nerve, thereby causing a slowing of nerve conduction over long segments of the nerve.
Nerves typically have different structures, depending upon their use. More particularly, there are generally three different types of nerve fibers: motor fibers, sensory fibers and autonomic fibers. The motor nerve fibers innervate muscle fibers and conduct impulses from the brain to the muscle. The sensory nerve fibers conduct impulses to the brain. Autonomic nerve fibers control the homeostatic sympathetic and parasympathetic nervous systems.
Autonomic nerve fibers have small diameters and generally cannot be effectively evaluated with NCS. However, motor nerve fibers and sensory nerve fibers can generally be effectively evaluated using NCS, thereby providing clinicians with an effective tool for the assessment of neuromuscular function.
More particularly, in an NCS evaluation of a motor nerve, the motor nerve is electrically stimulated and the motor nerve fibers are assessed by measuring the electrical response of the muscle innervated by the stimulated nerve. Correspondingly, in an NCS evaluation of a sensory nerve, the sensory nerve fibers are assessed by electrically stimulating the nerve and measuring the response of the sensory nerve fibers (i) at a fixed distance from the point of stimulation, or (ii) at an anatomical site remote from the point of stimulation.
Electrical stimulation depolarizes a short segment of the nerve (motor or sensory) at the point of stimulation. If this electrical depolarization exceeds a certain threshold, an action potential impulse is initiated. This action potential impulse propagates along the nerve, both distally and proximally, from the point of stimulation.
The aforementioned action potential impulses have different forms and characters which will hereinafter be discussed in greater detail. For the sake of convenience, these action potential impulses will be discussed in the context of motor nerves and an NCS evaluation of such motor nerves, although they also have application to sensory nerves and an NCS evaluation of such sensory nerves.
Distally-propagating nerve impulses (also known as “orthodromic” impulses) reach the muscle and depolarize the muscle fibers, typically causing a response or “twitch” in the muscle. This electrical activity of the muscle is measured as a compound muscle action potential (CMAP). The muscle responses caused by the distally-propagating impulses are typically referred to as “early waves” or M-waves. The time from the application of the stimulus to the onset (i.e., first deflection from baseline) of the response waveform is generally referred to as the “latency” of the CMAP. This latency is a measure of the conduction velocity of the fastest fibers in the nerve. The amplitude of the waveform is proportional to, and is a measure of, the number of muscle fibers that are innervated by the nerve fibers. The latency and amplitude of the CMAP are used to assess the distal segment of the nerve. A delayed latency and low amplitude are generally indicative of a neuropathy. Normally, the latency and morphology of the M-waves remain unchanged from one stimulus to the next.
Impulses propagating proximally along the axons (also known as “antidromic” impulses) reach the motor neuron cell bodies in the anterior horn of the spinal cord. In a small (and random) fraction of the neurons, the neuron depolarizes again (i.e., essentially “backfiring”), resulting in a new distally-traveling impulse (this is sometimes referred to as “back propagation”). The muscle responses due to these back-propagating impulses are generally referred to as F-waves. The F-waves travel through a longer segment of the nerve and are therefore more sensitive to systemic changes in the nerve fibers. However, unlike the aforementioned M-waves, the latency and morphology of the F-waves typically vary from one stimulus to the next, and typically comprise random features. In fact, F-waves generally have noise-like or variable features associated with them. Furthermore, due to the participation of only a small fraction of neurons in the creation of the F-waves, the amplitudes of F-waves are also much smaller than the amplitudes of M-waves. Thus, while F-waves have substantial utility in evaluating certain kinds of neuropathy, they can also be significantly more difficult to analyze than M-waves, due to their highly variable features and low amplitude.
Sometimes, a few neurons have a high probability of backfiring, and the repeated backfirings of these neurons produce features that have a similar morphology across recordings. These backfirings are frequently referred to as “Repeaters”. The latency of these Repeater waves is generally similar to that of other F-waves.
In addition to the aforementioned F-waves and Repeater waves, other action potential impulses are also generated proximal to the point of stimulation but not in the motor neuron itself. More particularly, normal or pathological axonal branching may exist in some nerves. These axonal branchings can produce action potential impulses that repeat with constant latency across recordings. These recurrent action potential impulses usually occur before the onset of the F-waves, and are commonly referred to as A-waves. A-waves are typically present in a significant percentage (i.e., approximately 50% or more) of the data recordings. These low amplitude A-waves are generally fairly consistent from stimulus to stimulus.
F-waves, Repeaters and A-waves may all be generated in response to the same stimulus.
The F-waves, Repeaters and A-Waves are sometimes collectively referred to as “late waves”. The fraction of the recordings in which a late wave feature is present is sometimes called the persistence of the feature. For example, A-waves have a higher persistence than Repeaters.
The recorded nerve response to a stimulus is typically called a trace. The ratio of traces in which variable F-waves exist relative to the total number of traces is sometimes referred to as the “F-wave persistence”. The earliest F-wave latency occurring in all traces having F-waves is frequently referred to as the “minimum F-wave latency”. Similarly, the mean F-wave latency over all traces with F-waves is frequently referred to as the “mean F-wave latency”.
The minimum F-wave latency, the mean F-wave latency, the F-wave persistence, the presence of A-waves, as well as other F-wave and A-wave parameters, and also the presence of Repeaters and their parameters, are frequently used to assess the proximal segment of the nerve. The delayed latency and low persistence of F-waves, and their parameters, and the presence of A-waves, are often particularly good indicators of a neuropathy. Thus, F-wave latency is frequently of significant interest to the clinician looking to assess neuromuscular function.
The highly variable morphology and very low amplitude of F-waves make latency calculations difficult due to complicating factors such as noise, power-line frequency interference (PFI) and baseline disturbance. Any algorithm that searches an entire trace for the purposes of detecting and assigning F-wave latency will generally be prone to error, since the algorithm may erroneously identify A-waves, noise, PFI and/or baseline disturbances as F-waves. Any algorithm that could correctly limit the search for F-wave latency in a trace to a limited time segment would significantly improve the accuracy of an automated F-wave latency calculation.